Recently, organic photoreceptor has become the principal type of electrophotographic photoreceptors in place of an inorganic photoreceptor. The reason being that organic photoreceptor has a wider selection range of a photosensitive material, superiority in environmental suitability and lower cost compared with an inorganic photoreceptor such as a selenium photoreceptor and an amorphous silicon photoreceptor.
Besides, digital image forming system using a LED or a laser as the light source is rapidly permeating in the field of a printer for personal computer and usual copying machine for reason of ease of image processing and development of a combined machine function. In addition, technique has been developed to progress the precision of the image and to form a high quality electrophotographic image. For example, technique is disclosed in which imagewise exposure is carried out by a laser beam having small spotting area for raising the dot density to form a high precision latent image, and the latent image is developed by a fine particle toner, c.f. Publication 1.
An organic photoreceptor is required for forming such the high quality image, which has high sensitivity and stable properties concerning the thermal environment.
Hitherto, for satisfying such the requirements of organic photoreceptor, the organic photoreceptor are constituted of a photosensitive layer in which the functions are separated by a charge generation layer and a charge transfer layer, and the charge transfer layer contains a large amount of a lower molecular weight charge transfer material having a molecular weight of approximately 500. However, in the charge transfer layer having such a constitution, the quality of the layer is easily degraded and the surface of the charge transfer layer is also easily contaminated by foreign matter. The surface of the photoreceptor tends to be contaminated by paper powder or toner components during the developing process, transferring unit and cleaning unit arranged around the photoreceptor. As a result of that, recurring image defects such as a black spotting (spots in a strawberry shape) and lacking of toner transfer tends to occur. Satisfactory sensitivity is difficult to obtain in a high speed copying machine in which the duration between the exposing process and the developing process is very short or under a low temperature and/or low humidity condition. As a result of that, dot image cannot be exactly reproduced and a fine line image tends to be easily degraded.
As a measure for solving such the problem, a method using a charge transfer material having high molecular weight is developed. For example, an organic photoreceptor has been reported in Publication 2, in which a charge transfer material having a bi-styryl structure with a molecular weight of 1,000 or more is used. Such a compound, however, cannot be uniformly dispersed in the binder resin when the compound is contained in the charge transfer layer since the compatibility of the compound with the binder resin tends to be insufficient. Consequently, the sensitivity becomes insufficient and damage such as cracking of the charge transfer layer tends to occur. Furthermore, a photoreceptor using a charge transfer material having a molecular weight of from 3,000 to 5,000 is reported in Publications 3 and 4. However, the residual potential tends to be raised on the photoreceptor using such the compound since the terminal of the compound is not blocked and the problem of the compatibility with the binder resin has not been sufficiently dissolved.
A photoreceptor is proposed in Publication 5, in the surface layer of which fluororesin particles are contained for preventing the contamination of the surface. However the use of the fluororesin particles in the surface layer tends to cause the image spreading. The mechanical strength of the surface layer also tends to be degraded and the surface of the photoreceptor is easily worn by the friction of the cleaning unit. Accordingly, a satisfactory image cannot be constantly provided.
Publication 1: Japanese Patent Publication Open to Public Inspection hereinafter referred to as JP O.P.I. Publication, No. 2001-255685.
Publication 2: JP O.P.I. Publication No. 3-149560
Publication 3: JP O.P.I. Publication No. 3-149560
Publication 4: JP O.P.I. Publication No. 5-25102
Publication 5: JP O.P.I. Publication No. 63-65449